1. Field of the Invention
This invention relates to strobo apparatus and more particularly to a control circuit efficiently flashing and stopping flashing in a strobo apparatus used in photographing or the like.
2. Related Background Arts
As well known, in a flash photography using a strobo apparatus, a flash discharge tube is made to flash toward an object to be photographed and the flash is stopped at the time when the most suitable exposure value is reached. Also, in order to control the flash of the flash discharge tube of the above mentioned strobo-apparatus, it has been general to connect a thyristor as a semiconductor switching device in series with a flash discharge tube to control switching on/off the conduction of the thyristor.
However, the conventional thyristor controlled type strobo circuit has in defects that an arc making circuit for switching on the thyristor and an arc extinction commutating circuit for switching off the thyristor are respectively required and the circuits are complicated and costly.
Therefore, in order to eliminate these defects, there is provided a strobo controlling circuit (See the specification of U.S. Pat. No. 4,839,686) using such gate controlling type switching device as a recently practiced insulated gate bipolar transistor (abbreviated as IGBT hereinafter) instead of the thyristor.
As compared with the conventional strobo controlling circuit using a thyristor, this strobo controlling circuit using a gate controlling type switching device has a feature that, as it uses a self-arc extinction type device which is a function of the device itself, no commutating circuit is required. However, in order to control such controlling device as this IGBT so as to be sufficiently conductive, it is necessary to apply to the gate a voltage of several 10's of volts which are higher than that of the thyristor.
As a means of obtaining such high gate voltage as these several 10's volts, there is already a means shown in a strobo controlling circuit using such IGBT as is disclosed in the specification of U.S. Pat. No. 4,839,686. In this controlling circuit, as shown in FIG. 16 incorporated herein a voltage boosting coil S is attached to a battery power source voltage boosting circuit 200 for high voltage charging a main condenser C so that an alternating current voltage induced in this coil S may be converted to a constant direct current voltage by a constant voltage circuit 201 comprising a transistor Q.sub.1, Zener diode ZD and smoothing condenser C.sub.1 of a large capacity to make a voltage of several 10's volts to be applied to the gate and this voltage may be applied to the gate of the IGBT. The above mentioned transistor Q.sub.2 is operated to be on by a transister Q.sub.3 switched on by a flash starting signal TRIGL issued from a control signal generating circuit 202. When a flash stopping signal STOPL is issued from the above-mentioned signal generating circuit 202, transistors Q.sub.4 and Q.sub.5 will be on and the above-mentioned transistors Q.sub.2 and Q.sub.3 will be off so that the application of the gate voltage to the IGBT may be removed.
Also, in this strobo controlling circuit, a trigger transformer T for applying a high trigger voltage to the trigger electrode of a discharge tube Xe is connected to the collector of the IGBT connected in series with the above-mentioned flashing discharge tube Xe so that, when the IGBT is conductive, the trigger condenser C2 will be charged and a high voltage will be induced in the trigger transformer T and will be applied to the trigger electrode.
On the other hand, a strobo controlling circuit using a well known FET (field effect type transistor) as a gate-controlling type switching device is also disclosed in the specification of U.S. Pat. No. 4,847,538 but, in this controlling circuit, too, exactly the same means as the means for obtaining the gate voltage of the above-mentioned IGBT is adopted for the gate voltage of the FET connected in series with the flashing discharge tube.
However, in a strobo controlling circuit using such gate-controlling type device as the above-mentioned IGBT or FET, as the voltage to be applied to the gate electrode is obtained by utilizing the strobo power source voltage boosting circuit 200 as described above, there are defects of complicating the circuit, a high number of component parts and a high cost that:
(1) An intermediate tap must be provided to attach the voltage boosting coil S to the secondary side winding of the voltage boosting transformer; PA1 (2) The constant voltage circuit 201 comprising the transistor Q.sub.1, Zener diode ZD and smoothing sub-condenser C.sub.1 of a large capacity must be provided in order that the alternating current voltage induced in the voltage boosting coil S may be made a stable voltage of several 10's volts to be applied to the gate; and PA1 (3) The gate circuit 203 comprising a plurality of high voltage resistant switching transistors Q.sub.2, Q.sub.3, Q.sub.4 and Q.sub.5 is necessary to retain the voltage to the gate electrode.
Further, after the main condenser C is charged, until a flash starting signal is input, the voltage necessary for the gate bias of the IGBT must be retained in the sub-condenser C.sub.1. Otherwise, the IGBT will not be on, therefore the discharge tube Xe will not flash or the IGBT will be likely to be broken by the short of the gate bias. Therefore, in consideration of being left for a long time, a condenser of less leakage than the main condenser and of a high precision and large capacity is required for the sub-condenser C.sub.1. The cost is high and a large fitting space is required. These are some of the problems encountered